Thermodynamic modelling of hydrocarbon-chains and light-weight supercritical solvents
by
James Edward Lombard
Thesis presented in partial fulfilment of the requirements for the Degree
of
MASTER OF ENGINEERING (CHEMICAL ENGINEERING)
in the Faculty of Engineering at Stellenbosch University
Supervisor Prof. J.H. Knoetze
Co-Supervisor/s Dr. C.E. Schwarz
March 2015 Stellenbosch University https://scholar.sun.ac.za
DECLARATION
By submitting this thesis electronically, I declare that the entirety of the work contained therein is my own, original work, that I am the sole author thereof (save to the extent explicitly otherwise stated), that reproduction and publication thereof by Stellenbosch University will not infringe any third party rights and that I have not previously in its entirety or in part submitted it for obtaining any qualification.
James Lombard February 2015
………………………. ………………………. Signature Date
Copyright © 2015 Stellenbosch University All rights reserved
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ABSTRACT
Long-chain hydrocarbons are of value to numerous lucrative industries. Due to the low volatility and close melting and boiling points of these solutes, traditional fractionation methods lack the required selectivity for separation and cause thermal degradation of the product.
This project investigates the feasibility of Supercritical Fluid Extraction (SFE) for processing these systems, with the primary objective of modelling the high-pressure vapour-liquid equilibrium (VLE) properties of hydrocarbon solutes with a light-weight solvent using a semi- empirical equation of state (EOS). Pure component vapour pressures and saturated liquid volumes are also investigated.
A thorough investigation into the phase behaviour of the n-alkanes, 1-alcohols, carboxylic acids and esters in light weight supercritical solvents CO2, ethane and propane revealed that the solute structure and temperature largely influence the solute solubility and process feasibility. Good selectivity amongst the various solutes was observed for all three solvents, but very high pressures were required for complete miscibility using CO2 (exceeding 30
MPa). The quadrapole moment of CO2 further leads to complexities in phase behaviour such as temperature and density inversions (CO2/alkanes and CO2/alcohols) and 3-phase regions within the operating range. Simple linear trends in pressure vs. carbon number and temperature were observed for all the considered series using ethane and propane and these solvents were thus selected for conducting the modelling for this study.
A thorough review of semi-emperical EOS models from literature revealed that the simple cubic equations of state (CEOSs) provide a promising modelling approach for SFE applications due to their simplicity, flexibility and reliability.
The simple Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) EOSs provide good correlation of vapour pressure (%AAD below 5 %) for all the series over a large carbon number range (up to nC20), provided a two parameter alpha function is used. A 3rd parameter in the volume dependence for Patel-Teja (PT) EOS provides considerable improvement over the PR and SRK EOSs for satureate liquid volume correlations of the non-polar solutes (alkanes and esters), but offers virtually no advantage for the more polar alcohols and acids. The CEOSs therefore suffer clear limitations in simultaneous representation of these saturation properties (vapour pressure and liquid molar volume) for the systems of interest.
Good correlations of high pressure binary VLE data were obtained using CEOSs available in the Aspen Plus ® simulator (% AAD in P, T and X2 generally below 1 % and ranging from 4
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to 12 % for Y2 for all series) provided that two binary interaction parameters (BIPs) are used in the model mixing rules, irrespective of the model used. Aspen Plus ® was further validated as a reliable thermodynamic tool by comparing model fits using the RK-ASPEN model with parameters obtained from the Aspen Plus ® data regression routine and computational methods used in self-developed MATLAB software. Very similar results were obtained for both computational methods, which encourages the use of Aspen Plus ® for process modelling in SFE applications.
A statistical sensitivity analysis into the relative effect and interactions between 6 modelling factors in applying the CEOSs revealed that the mixing rules, temperature and solute structure had the largest effect on the correlation of the high pressure VLE, with the pure component limit having negligible effect once BIPs are fitted to data. A significant interaction was, however, observed between the pure component model and the solute structure and temperature, which suggest that accurate correlation of mixture VLE does not solely rely on appropriate mixing rule selection, but also the pure model.
Binary interaction parameters (BIPs) in model mixing rules were found to become intercorrelated when more than one are used, greatly impeding the development of generalized correlations. BIPs were also found to be sensitive to the pure component limit (alpha function and pure constants used), the temperature, the combining rules used and possibly the fluid density. These factors should all be taken into account systematically for developing generalized correlations which therefore fell outside the scope of this study. Recommendations were, however, made on how the MATLAB software developed in this study can be used to both expand the size of the statistical analysis already conducted into relevant modelling factors and to develop new generalized correlations for BIPs and new mixing rules.
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OPSOMMING
Lang-ketting koolwaterstowwe is van waarde in talle winsgewende industriële toepassings. Vanweë die lae vlugbaarheiden ooreenstemmende kook- en smeltpunte van hierdie molekules, toon tradisionele fraktioneringsmetodes nie die nodige selektiwiteit vir ekstraksie nie en veroorsaak bonop termiese degradering van die produk.
Hierdie projek ondersoek dus die lewensvatbaarheid van superkritiese ekstraksie vir die prosesering van hierdie sisteme, met primêre fokus op die modellering van die hoë-druk damp-vloeistof ewewig eienskappe van koolwaterstowwe opgelos in ‘n lae-massa oplosmiddel met gebruik van ‘n semi-empiriese toestandsvergelyking. Suiwer-komponent dampdrukke en versadigde vloeistof volumes word ook ondersoek.
‘n Deeglike ondersoek na die fasegedrag van die n-alkane, 1-alkohole, korboksiel-sure asook esters in lae-massa superkritiese oplosmidds CO2, etaan en propaan toon dat die struktuur van die opgeloste stof en die temperatuur ‘n groot invloed het op die oplosbaarheid en proses lewensvatbaarheid. Goeie selektiwiteit tussen die verskillende koolwaterstowwe was waargeneem vir al drie oplosmiddels, alhoewel baie hoë drukke nodig was vir totale vermenging van die fases in CO2 (hoër as 30 MPa). Die quadrupool moment van CO2 veroorsaak verder ongewenste kompleksiteite in fase gedrag soos temperatuuren digtheid inversies (CO2/alkane en CO2/alkohole) en 3-fase-gebiede in die bedryfs-kondisies. Eenvoudige lineêre tendense in druk tenoor die koolstofnommer van die opgeloste stof asook die temperatuur was waargeneem vir al die ondersoekte koolwaterstof reekse in etaan en propaan en hierdie oplosmiddels was dus gekies vir die modellering vir hierdie studie. n’ Deeglike oorsig van semi-empiriese toestandsvergelykings uit die literatuur het getoon dat die eenvoudige kubiese toestandsvergelykings ‘n belowende modelleringsbenadering bied vir superkritiese ekstraksie toepassings vanweë hul eenvoudigeid, buigsaamheid enbetroubaarheid.
Die eenvoudige Peng-Robinson (PR) en Soave-Redlich-Kwong (SRK) toestandsvergelykings bied goeie korrelasie van suiwer dampdruk (foute laer as 5 %) vir alle koolwaterstowwe oor ‘n groot koolstofnommer gebied (tot by nC20), met die voorwaarde dat ‘n 2 parameter alpha funksie gebruik word. ‘n 3rde parameter in die volume afhanklikheid van die Patel-Teja (PT) toestandsvergelyking bied ‘n beduidende verbetering in die passing van die versadigde vloeistof volume vir die nie-polêre koolwaterstowwe (n-alkane en die esters), maar bied geen voordeel vir die meer polêre alkohole en karkoksiel sure nie. Die kubiese modelle toon dus duidelike beperkings vir die gelyktydige voorstelling van hierdie versadigingde eienskappe (dampdruk en vloeistof volume) vir die sisteme van belang.
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Goeie korrelasie van hoë druk binêre damp-vloeistof ewewig data was verkry deur gebruik van die kubiese toestandsvergelykings beskikbaar inAspen Plus ® (fout in P, T en X2 tipies laer as 1 % en van 4 tot 12 % vir Y2 vir alle sisteme), met die voorwaarde dat 2 binêre interaksie parameters gebuik word in die model mengreëls, onafhanklik van die model. Aspen Plus ® was verder bekraktig as ‘n betroubare termodinamiese hulpmiddel deur model passings te vergelyk met die RK-ASPEN model tussen gevalle waar parameters verkry is deur die beskikbare regressie metode in Aspen Plus ® en metodes gebruik in self-ontwikkelde MATLAB sagteware. Eenderse resultate was verkry vir beide berekeningsmetodes, wat die gebruik van Aspen Plus ® vir prosesmodellering in superkritiese ekstrasie toepassings aanmoedig.
‘n Satistiese sensitiwiteits analise op die relatiewe effek en interaksies tussen 6 modelleringsfaktore in die toepassing van die kubiese toestandsvergelykings het gevind dat die mengreëls, temperatuur en die stuktuur van die opgeloste stof die grootste effek op die korrelasie van hoë druk binêre damp-vloeistof ewewig het, met ‘n weglaatbare effek vandie suiwerkomponent limiet waargeneem sodra binêre interaksie parameters gepas is aan data. ‘n Beduidende interaksie was wel waargeneem tussen die suiwerkomponent model en die struktuur van die opgeloste stof asook die temperatuur, wat daarop dui dat akurate korrelasie van mengsel damp-vloeistof ewewig nie slegs afhanklink is van ‘n gepaste keuse van mengreëls nie, maar ook die suiwer-komponent model.
Binêre interaksie parameters in die model mengreëls ondergaan inter-korrelasie wanneer meer as een interaksie parameter gebruik word, wat die ontwikkeling van algemeen toepaslike korrelasies grotendeels belemmer. Binêre interaksie parameters was ook bevind om sensitief te wees tot die suiwer component limiet (alpha funksie en suiwer konstantes wat gebruik is), die temperatuur, die kombineringsreëls en moontlik die vloeistof digtheid. Hierdie faktore moet dus almal sistematies in ag geneem word wanneer algemeen toepaslike korrelasies ontwikkel word, wat dus buite die omvang van die huidge studie val. Aanbevelings was wel gemaak vir hoe die MATLAB sagteware ontwikkel vir hierdie studie gebruik kan word om beide die betaande statistiese analise uit te brei, asook nuwe korrelasies vir binêre interaksies parameters en nuwe mengreëls te ontwikkel.
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ACKNOWLEDGEMENTS
The financial assistance of the National Research Foundation (DAAD-NRF) towards this research is hereby acknowledged. Opinions expressed and conclusions arrived at, are those of the author and are not necessarily to be attributed to the DAAD-NRF.
Aspen Plus ® is a registered trademark of Aspen Technology Inc.
A special word of gratitude is conveyed to the following people, without whom the completion of this project would not have been possible:
• My supervisors Dr. C.E. Schwarz and Prof. J.H. Knoetze for their continued support and encouragement in pursuing my ideas throughout the duration of the study
• Dr. Christo Crause at SASOL for providing useful tips regarding the binary VLE calculations performed in the developed MATLAB software
• My parents and brother for their unconditional love, encouragement and patience
• Fellow researchers at the Separations Technology group at Stellenbosch University for providing a stimulating working environment
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TABLE OF CONTENTS DECLARATION ...... i ABSTRACT ...... ii OPSOMMING ...... iv ACKNOWLEDGEMENTS ...... vi 1. INTRODUCTION ...... 1 1.1 The feasibility of SFE ...... 1 1.1.1 Systems ...... 1 1.1.2 Traditional methods ...... 2 1.1.3 SFE as alternative ...... 2 1.1.4 Summary ...... 3 1.2 The role of thermodynamic modelling within SFE ...... 3 1.2.1 Experimentation and databases ...... 4 1.2.2 Correlation, prediction and simulation ...... 4 1.3 Project objectives ...... 5 1.4 Thesis layout ...... 7 2. BINARY PHASE DIAGRAMS AND THE CRITICAL REGION ...... 10 2.1 The supercritical phase ...... 10 2.1.1 General critical point theory ...... 10 2.1.2 Physical properties of supercritcal fluids (SCFs) ...... 15 2.1.3 The mechanism of Supercritcal Fluid Extraction (SFE) ...... 19 2.2 Binary phase diagrams ...... 19 2.2.1 The general phase equilibrium problem ...... 19 2.2.2 Binary phase diagram definitions ...... 21 2.2.3 Binary phase behaviour: Type 1 to 5 ...... 23 2.2.4 Studies on homologous series ...... 28 2.3 Summary of challenges ...... 31 2.3.1 Critical point complexities ...... 31 2.3.2 System complexities ...... 32 2.3.3 Proposition for addressing the challenges ...... 34
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2.4 Conclusions ...... 34 3. SYSTEMS INVESTIGATED ...... 37 3.1 Solvents and solutes considered ...... 37 3.2 Selectivity for functional group (energetic differences) ...... 38 3.3 Selectivity for carbon backbone length (size and mass differences) ...... 41 3.4 Selectivity for the side-branching ...... 43 3.5 Influence of temperature ...... 43 3.6 Solvent and solute selection for modelling ...... 48 3.7 Conclusions ...... 49 4. EQUATIONS OF STATE FOR APPROACHING THE CRITICAL REGION ...... 52 4.1 The virial equation of state ...... 52 4.1.1 Theoretical low density limit for mixing rules ...... 54 4.2 The cubic Van der Waals equations of state ...... 54 4.2.1 Volume dependence ...... 55 4.2.2 Volume translation ...... 57 4.2.3 Temperature dependence (Alpha function) ...... 57 4.2.4 Mixing rules ...... 59 4.2.5 Binary interaction parameters ...... 60
4.2.6 EOS/Gex mixing rules ...... 61 4.3 Polymer-chain molecular models ...... 67 4.3.1 PHCT ...... 68 4.3.2 SPHCT ...... 72 4.3.3 PSCT ...... 73 4.4 SAFT molecular models ...... 74 4.4.1 Original SAFT (Huang and Radosz) ...... 75 4.4.2 PC-SAFT ...... 78 4.4.3 Simplified PC-SAFT ...... 79 4.4.4 SAFT-CP ...... 79 4.4.5 Numerical pitfalls of the SAFT models ...... 81 4.4.6 SAFT + Cubic ...... 82
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4.5 Group contribution methods ...... 84 4.5.1 PPR78 ...... 85 4.5.2 GPSCT ...... 86 4.5.3 GSPHCT ...... 87 4.5.4 PT-GC ...... 87 4.5.5 GC-SAFT ...... 88 4.5.6 GC-PC-SAFT ...... 88 4.5.7 GC-EOS by Skjold-JØrgensen ...... 88 4.6 The Crossover approach ...... 90 4.6.1 Crossover and cubic models ...... 90 4.6.2 Crossover and molecular models ...... 91 4.7 Concluding remarks and modelling approach selection for this study ...... 91 5. MODELLING METHODOLOGY ...... 94 6. PURE COMPONENTS...... 97 6.1 Thermodynamic theory: Phase equilibrium for a pure component ...... 97 6.2 Models investigated ...... 100 6.3 Reduction of data ...... 101 6.4 Pure component constants ...... 102 6.5 Obtaining model parameters ...... 104 6.5.1 Primary Soave parameter ...... 104 6.5.2 Empirical critical compressibility of the PT EOS ...... 107 6.5.3 Additional empirical alpha function parameters ...... 109 6.6 Vapour pressure and saturated liquid volume results ...... 109 6.6.1 n-Alkanes ...... 110 6.6.2 1-Alcohols ...... 114 6.6.3 Carboxylic Acids ...... 116 6.6.4 Methyl Esters ...... 118 6.7 Influence of regression weights ...... 120 6.8 Conclusions ...... 122
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7. MODELLING WITH A COMMERCIAL PROCESS SIMULATOR: ASPEN PLUS ® 125 7.1 Systems modelled ...... 126 7.2 Models investigated ...... 127 7.3 Reduction of data ...... 130 7.3.1 Data smoothing ...... 130 7.3.2 Regression ...... 130 7.4 Results ...... 131 7.4.1 Overall results: Ethane ...... 131 7.4.2 BIP values: Ethane systems...... 133 7.4.3 Ethane plots ...... 134 7.4.4 Overall results: Propane ...... 137 7.4.5 BIP values: Propane systems ...... 139 7.4.6 Propane plots ...... 140 7.5 Including data in the critical region ...... 142 7.6 Qualitative effect of BIPs ...... 144 7.7 BIPs vs. Solute carbon number ...... 147 7.8 Conclusions ...... 149 8. STATISTICAL SENSITIVITY ANALYSIS FOR BINARY VLE MODELLING ...... 151 8.1 Thermodynamic theory: Phase equilibrium of a mixture ...... 151 8.2 Reduction of data ...... 153 8.3 Factor levels ...... 154 8.4 Results of statistical analysis ...... 162 8.4.1 Normal probability plot of residuals ...... 162 8.4.2 Statistical concepts ...... 163 8.4.3 Main effects ...... 166 8.4.4 Interaction effects ...... 168 8.4.5 Optimization ...... 173 8.4.6 The effect of the pure component limit on BIPs ...... 177 8.5 Conclusions ...... 181
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9. COMPARISON OF SELF-DEVELOPED SOFTWARE WITH RESULTS FROM ASPEN PLUS ...... 184 9.1 Models® ...... 185 9.2 Model fit results ...... 187 9.2.1 Comparison of different computational techniques ...... 187 9.2.2 Overall model comparison ...... 189 9.3 BIPs vs. CN for PRSV-KM ...... 189 9.4 Conclusions ...... 192 10. CONCLUSIONS ...... 195 10.1 Objective 1 : Review theory on critical points, binary phase diagrams and obtaining the required property information for SFE applications ...... 195 10.2 Objective 2 : Review interesting phase behaviour of systems considered ...... 196 10.3 Objective 3 : Give overview of semi-empirical EOS models ...... 196 10.4 Objective 4 : The pure component limit ...... 197 10.5 Objective 5 : Determine capabilities of commercial process simulator ...... 197 10.6 Objective 6 : Investigate trends in BIPs for developing generalized correlations 198 10.7 Objective 7 : Investigate the effect and relative importance of modelling factors for binary VLE at high pressure ...... 198 10.8 Objective 8 : Investigate the effect of different computational procedures on the results 199 11. RECOMMENDATIONS AND FUTURE WORK ...... 201 11.1 Possible contributions and motivation for upgrade of current study ...... 201 11.2 Future work ...... 202 12. REFERENCES ...... 204 13. NOMENCLATURE ...... 219 13.1 List of symbols ...... 219 13.2 Greek symbols ...... 223 13.3 Superscripts ...... 224 13.4 Subscripts ...... 225 13.5 Value of constants ...... 226 13.6 Abbreviations ...... 226
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APPENDIX A: Working Equations ...... 228 A.1 Pure models ...... 228 A.1.1 Peng-Robinson [1] ...... 228 A.1.2 Patel Teja [2] ...... 229 A.1.3 SRK [3] ...... 230 A.1.6 Alpha functions ...... 231 A.1.4 UNIFAC [7] ...... 232 A.1.5 NRTL [8] ...... 234 A.2 Mixing rules ...... 234 A.2.1 Van der Waals mixing rules ...... 234 A.2.2 Wong-Sandler mixing rules ...... 235 A.3 Expressions for pure component fugacity ...... 236 A.3.1 Peng Robinson [1] ...... 236 A.3.2 Patel Teja [2] ...... 237 A.3.3 SRK [3] ...... 237 A.4 Expressions for fugacity of component in solution ...... 237 A.4.1 Peng-Robinson ...... 237 A.4.2 Patel Teja ...... 238 A.4.4 SRK ...... 240 References ...... 240 APPENDIX B: Algorithms and numerical methods ...... 242 B.1 Root solving ...... 242 B.2 Pure Components ...... 244 B.3 Binary Mixtures ...... 246 B.4 Validation of the code ...... 248 References ...... 251 APPENDIX C: Pure component constants used ...... 252 C.1 DIPPR ...... 252 C.2 Constantinou & Gani group contribution method ...... 254 C.3 Aspen Plus ® (Pure 20 database) ...... 257
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References ...... 259 Appendix D: Important theoretical developments applicable to high pressure phase equilibrium ...... 260 D.1 The ideal gas law ...... 260 D.2 The kinetic theory of gases ...... 261 D.3 Intermolecular forces and potential-energy functions ...... 264 D.3.1 Attractive potential-energy functions ...... 266 D.3.2 Repulsive potential functions ...... 271 D.3.4 Combined potential functions ...... 272 D.3.5 Quasi-chemical forces ...... 276 D.4 The Van der Waals equation of state ...... 278 D.4.1 The principle of corresponding states ...... 281 D.4.2 VLE from the Van der Waals equation ...... 283
D.4.3 The critical compressibility factor (Zc) ...... 285 D.5 Molecular models and perturbation theory ...... 286 D.5.1 The Boltzmann Distribution ...... 286 D.5.2 The Partition function ...... 287 D.5.3 The Radial Distribution Function (Pair correlation function) ...... 289 D.5.4 Perturbation theory ...... 291 D.5.5 Evaluation of reference and perturbation terms ...... 292 References ...... 294 APPENDIX E: Additional figures and tables ...... 296 E.1 ASPEN regression: BIP vs. CN plots for each case ...... 296 E.1.1 Ethane/n-Alkanes ...... 296 E.1.2 Ethane/1-Alcohols ...... 299 E.1.3 Ethane/Carboxylic Acids ...... 302 E.1.4 Ethane/Methyl Esters ...... 305 E.1.5 Propane/n-Alkanes ...... 308 E.1.6 Propane/1-Alcohols ...... 311 E.1.7 Propane/Carboxylic Acids ...... 314
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E.2 ASPEN regression: Parameter values ...... 317 E.3 Pure components ...... 320 E.3.1 n-Alkanes ...... 320 E.3.2-Alcohols ...... 322 E.3.3 Carboxylic Acids ...... 325 E.3.4 Methyl Esters ...... 328 E.3.5 Pure parameters ...... 331
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1. INTRODUCTION
This project involves thethermodynamic modelling of the high-pressure binary vapour-liquid equilibrium (VLE) properties of long-chain hydrocarbon solutes (carbon number greater than 10) from different homologous series in solution with a supercritical solvent. This property information is crucial in the design of a super-critical fluid extraction (SFE) process, which aims to fractionate certain ranges of hydrocarbon-chain molecules into narrow cuts of similar structural features from a complex mixture. The data for this study has been measured using the facilities at Stellenbosch University and include the n-alkane, 1-alcohol, methyl and ethyl ester, as well as the carboxylic acid family in light-weight solvents, ethane, propane and CO2 [1-17].
1.1 The feasibility of SFE
This section briefly overviews the feasibility of SFE for fractionating the systems investigated for this study. The value of the systems, the shortfalls of traditional methods and the viability of SFE is discussed.
1.1.1 Systems
Complex hydrocarbon-chain mixtures are encountered in a wide range of both naturally and synthetically occurring matrices and their processing is of interest to numerous lucrative industries [18]. Synthetic paraffin waxes in the carbon number range 30 – 300, for example, are present in crude oil reserves and are also the primary constituents of the Fischer Tropsch petro-chemical process effluent stream [19]. Fractionation of these mixtures into narrower cuts of similar carbon backbone lengths are of interest, amongst others, to the manufacture of candles, coatings in the printing, paper and food industries as well as additives to improve insulation properties of construction materials [20]. Long-chain alcohols play an important role in the production of cosmetic and detergent range (carbon number 12 to 16) products and are typically naturally sourced, converted from natural products or synthesized from the oxidation of other long-chain hydrocarbons [3, 21, 22]. The processing of fats and oils as found naturally in plant and animal materials is also of considerable commercial value to the food, cosmetic, pharmaceutical and oleo-chemical industries [23]. These oils and fats are comprised of complex mixtures of lipids such as triglycerides, free fatty acids, phospholipids, glycolipids, sterols and other fat soluble components [23]. Often in processing the fatty acids from a feedstock, they are converted to their corresponding methyl or ethyl ester and subsequently fractionated [6].
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1.1.2 Traditional methods
Traditional fractionation methods for the mentioned systems include distillation, liquid-liquid extraction, adsorption, fractional crystallization and membrane technologies. These technologies are well-established in industry, require lower operating pressures, and have few safety concerns [24]. Due to the low-volatility of the long-chain solutes, these technologies require high operation temperatures which lead to thermal degradation of the product [18]. They also have insufficient selectivity for the close melting and boiling points of these solutes. Organic solvents such as hexane and toluene, as typically used in the liquid-liquid extraction of fats and oils, are furthermore facing government restrictions due to safety and environmental concerns [23].
1.1.3 SFE as alternative
The use of supercritical solvents is emerging as a feasible alternative for treating such systems. Close to the solvent critical temperature, the fluid shows large variations in density with small changes in temperature and pressure. Solubility is a strong function of density, which allows solvents to be selectively tuned for fractionation of certain solute ranges with small changes in the operating conditions and enables dissolving capabilities approaching those of liquids. Low weight super-critical solvents are volatile gases at atmospheric conditions, which further leads to simple separation from the final extract by either pressure reduction or temperature rise, with virtually negligible solvent residue in the product [19, 23].
The most common method for fractionating synthetic waxes is currently short path distillation (SPD) [19]. Operating pressures in the 0.1 – 10 Pa range can be reached, which is much lower than standard vacuum distillation units and low enough to prevent thermal degradation of most solutes [19]. Nieuwoudt et al. [25] compared the technical feasibility of SFE with SPD for wax fractionation and found that SFE gave much narrower cuts and that SPD gave a yellow colouration of the product. Crause et al. [19] found that static crystallisation had higher up-front capital costs than both SFE and SPD and that through efficient heat integration, SFE was technically and financially the more viable technology for fractionating long chain paraffins with carbon backbone exceeding nC45. Most petro-chemical plants also have ethane and propane available on site as cracker feedstock, as well as low pressure steam utilities, which improves the feasibility of integrating SFE with existing process units and possible re-processing of effluent streams [19].
In addition to providing solutions for the inadequacy of traditional methods, the unique properties of supercritical solvents also allow for new niche-markets to be exploited [24].
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Recent developments include removing pesticides from phytochemicals and neutraceuticals, dry cleaning and degreasing of precision parts in electronics, dying of textiles and use as mobile phases in chromatography. Due to the high solvent selectivity, novel products in the extraction and purification of nutraceuticals, food supplements, active ingredients of pharmaceuticals, as well as application as a polymerization media are also being developed [24].
1.1.4 Summary
The unique characteristics of supercritical fluids (SCFs) have spurred immense research activity over the last two decades, but this activity is currently not proportional to the number of industrial applications [24, 25]. The general process complexities lead to case specificdesign for large scale applications, requiring substantial R&D efforts [18, 27]. This situation makes reliable cost estimates difficult, but recent reviews suggest that process economics improve substantially as the throughput of the process increases [18, 26]. Continuous operation or long-duration batches further allows for substantial savings on manpower [26].
It is believed that the immense research efforts in this field over the last 25 years will be able to accommodate the flurry of new applications currently waiting in the pipe-line, while simultaneously soothing growing environmental concerns regarding traditional solvents. With realistic capital cost and maintenance estimation, as well as optimized design and operation, SFE has the full potential to emerge as a prominent separations technology in its own right, on all scales of industry.
1.2 The role of thermodynamic modelling within SFE
The design of a SFE process involves the following general steps:
• Obtain the required property information • Develop a process model for the fractionation columns • Design the fractionation process
This project focuses exclusively on the first step, namely obtaining a reliable source of property information for incorporation into the design of a SFE process. According to O’ Connell et al. [28], there are three main sources of property and phase equilibrium information available to a process engineer, including:
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- Self-conducted experiments - Databases of published values from literature, and - Estimation methods using correlation, prediction or computation.
1.2.1 Experimentation and databases
In most design circumstances conducting rigorous experiments is not feasible given time and resource requirements. Recent revolutions in computation and information science have allowed for many companies to access vast electronic databases of property information. These databases may be easily searched and updated, but the increasing demand of global industrial applications seems to always exceed the rate of data acquisition. According to O’ Connell et al. [28], as of March 2009 the CAS (Chemistry Abstracts Service) registry contains 45 000 000 organic and inorganic substances, with a total of nearly 61 000 000 chemical sequences. Despite dealing with the problem of measuring data for the infinite combination of mixtures at the appropriate conditions, a substantial hurdle in managing this body of information is also determining the quality of the data. Errors of consistency, tabulation and omission could cause large problems in application for design purposes. It is therefore clear that obtaining the relevant property values solely through empirical means is not sustainable.
1.2.2 Correlation, prediction and simulation
The methods of correlation and prediction are in the form of mathematical property models within which substances are defined by a set of parameters specific to the model. The different forms which such a model may take can be related to the level of empiricism involved in describing the system.
If the property model is generated through curve-fitting all parameters to sparse experimental data using polynomials, log-log plots, time series analysis or ANOVA methods, the model may be regarded as a purely empirical model. Empirical models are generally only capable of approximate interpolation between points in the design space, with no reliable prediction capabilities outside the system conditions from which the data were obtained.
A purely theoretical model is based entirely on pre-established knowledge of the system components, conditions and the fundamental physical principles involved, with no parameters arbitrarily regressed to data. Such a model should ideally have a single parameter set for each chemical compound, rather than working with different parameter sets for estimating different properties; or as may also be necessary, different parameter sets for different operating
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conditions for the same property [29]. Theoretical models should also have a functional form with minimal loss of accuracy upon differentiation with respect to any process variable. Theoretical models are generally derived from the disciplines of quantum mechanics and statistical mechanics, but can currently only provide precise predictions for highly simplified systems in which all initial conditions are known and intermolecular interactions are essentially negligible. Such systems are hardly ever encountered in engineering practice.
In between the empirical and theoretical approaches there is an approach which O’ Connell et al. [28] refers to as “enlightened empiricism.” This approach uses rigorous equations from chemical theory along with correlations and parameters adjusted to fit data. These models are referred to as semi-empiricaland have been the dominant method for obtaining required property information inseparation process design.
Advances in processing power may soon see computational simulation becoming the primary method for obtaining property information in design applications, but the immense scale and complexity of chemical systems has so far prevented this transition.
1.3 Project objectives
The primary aim of this project is to establish an effective semi-empirical thermodynamic modelling methodology to obtain the required property information for designing a supercritical fluid extraction (SFE) process. This requires not only an understanding of thermodynamic models, but also of unique features of the critical point and the phase behaviour of the systems in terms of intermolecular interactions. The objectives of this project are therefore divided into theoretical and modelling objectives:
Theoretical objectives
1) Get acquainted with relevant theory regarding the critical point, binary phase diagrams and the challenges in obtaining the required property information for SFE applications 2) Gain a thorough understanding of how structural features of the solute such as functional end-group, carbon backbone and isomerism (side-branching), as well as temperatureinfluence the phase behaviour, solvent selection and feasibility for a SFE process. 3) Review existing thermodynamic models for obtaining the required property information and make an appropriate selection for SFE applications.
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Modelling objectives
4) Determine the capabilities of the selected modelling approach in representing the pure component vapour pressure and saturated liquid volume for the components of interest to SFE applications. 5) Determine the capabilities of commercial process simulators to model the high- pressure binary VLE data for asymmetric systems of hydrocarbon-chain solutes in a supercritical solvent, approaching the mixture critical point. 6) Investigate model parameters for trends with solute structure for possible developmentof generalized correlations. 7) Determine the effect and relative importance of factors such as the pure component limit, the mixing rules and the system conditions on the thermodynamic modelling of high pressure VLE of the asymmetric binary systems of interest to SFE applications. 8) Determine the effect of different computational techniques on the final results.
The outcomes of the first three theoretical objectives involve making appropriate selections regarding the computational procedure to be used, systems (solutes and solvents) considered and thermodynamic models to be investigated for this study.Objectives 4 through 8 involve conducting the thermodynamic modelling of the selected systems using the selected modelling approach and numerical procedure. When a typical phase diagram is considered, the different regions are:
• Two-phase equilibrium regions • Compressed liquid region • Superheated vapour region • Solid region • The near critical region • The above critical region
The regions to be modelled for this study are the high pressure vapour-liquid equilibrium
(VLE) properties, namely T, P, {X}, {Y}, just above the critical temperature of the solvent at reduced temperature (Tr = T/Tc) of 02 – 1.3, and approaching the mixture critical point, which is where solubility is deemed to be most feasible. Pure component vapour pressure and saturated liquid densities will also be investigated.
Even though the focus of the project is primarily on obtaining the relevant property information through thermodynamic modelling, it is noted that the study strives for a holistic view by placing property modelling in the wider context of designing a SFE process.
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1.4 Thesis layout
A thesis layout is subsequently given for addressing the project objectives. Chapter 2 addresses project objective 1 by discussing the unique characteristics of the supercritical phase and the general theory behind binary phase diagrams according to the classification of Van Konynenburg and Scott [30]. Unique challenges for obtaining property values in the high pressure region approaching the critical point are also discussed and a computational procedure is proposed for addressing the numerical challenges.
Chapter 3 addresses project objective 2 by investigating the phase behaviour of the systems considered for this thesis, with emphasis on process feasibility.Solvents and solutes are then selected for conducting the modelling for this study.
Chapter 4 addresses project objective 3through an overview of semi-empirical equations of sate (EOS) for high-pressure applications, with emphasis on the near-critical region. Model families considered include the virial EOS, the cubic equations of state (CEOS), the molecular models for polymer chains (Perturbed Hard Chain Theory and related models), the Statistical Association Fluid Theory (SAFT) models for association molecules, the group contribution methods and the crossover approach. An appropriate approach is thenselected for this study.
Chapter 5 outlines the precise modelling methodology followed for addressing project objectives 4 through 8 in the ensuing chapters using the selected approach from Chapter 4.
Chapter 6 addresses project objective 4 by investigating the representation of the pure component vapour pressure and saturated liquid volume by the selected modelling approach. Appropriate pure component model parameters are also obtained prior to conducting mixture modelling.
Chapter 7 addresses project objective 5 by investigating the ability of current simulation packages to model the high pressure VLE of the selected binary systems using the general modelling approach chosen from Chapter 4. Aspen Plus ® is used for this investigation due to its wide application in industry and academia, as well as the many property models it has available. Project objective 6 is also addressed in this chapter by investigating binary interaction parameters (BIPs) in the model mixing rules for trends with solute carbon number.
Chapter 8 addresses project objective 7 by investigating important factors in the chosen modelling approach using a design of experiments (DOE) statistical sensitivity analysis. 6 important modelling factors are identified, each at two levels, implying 26 = 64 separate
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treatments (modelling combinations). The first 4 factors are model dependent and include the temperature dependence of the model, the volume dependence, the source of the pure component constants and the mixing rules used. The remaining two factors are system dependent and include the operating temperature and the terminal functional group of the solute.The sensitivity of BIPs to modelling factors involving the pure component limit is also investigated.
Chapter 9 addresses project objective 8 by comparing results from self-developed MATLAB software with those obtained from Aspen Plus® using the same model. Project objective 6 is also addressed in this chapter through investigating the influence of combining ruleson trends of BIPs with solute carbon number. Chapter 10 and 11 summarize the conclusions, recommendations and suggested future work from the study. The thesis layout is summarized in Figure 1-1.
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Chapter 2: Critical region and Chapter 3: Systems investigated binary phase diagrams - Solvents and solutes considered -The super-crtical phase - Influence of functional end-group; - Binary phase diagrams carbon-backbone, side-branching, - Challenges in obtaining and temperature on phase behaviour property values - Solvent and solute selection
Chapter 5:Modelling methodology Chapter 4: Review of EOS models - Pure components -Virial EOS, cubic EOSs, PHCT ,SAFT, - Aspen Plus ® group contribution methods, Crossover - DOE sensitivity analysis - Model selection is made - Comparison of MATLAB with Aspen Plus ® Chapter 6: Pure component properties - Vapour pressure and sat. liq. vol. Chapter 7: Aspen Plus ® - Pure constants used - Correlation of high-pressure VLE - Model parameters used - Trends in BIPs - Volume and temperature dependence
Chapter 8: DOE sensitivity analysis - 6 modelling factors, each at 2 levels: - 1) Volume and 2) temperature dependence, 3) pure constants and 4) mixing rules used, 5) system temperature, 6) solute structure - Main effects and interactions investigated
Chapter 9: Comparison of MATLAB and Aspen Plus ® - Different computational techniques, same model - Effect of combining rules onBIP behaviour
Chapter 10 and 11: Conclusions, recommendations and future work
Figure 1-1 Diagram of thesis layout
Appendix A includes all of the working equations used and Appendix B gives the computational procedures used in the MATLAB software developed for conducting this study. Appendix C gives all of the pure constants used in the different sections of the project. Appendix D gives a chronological overview of important theoretical aspects in thermodynamic model development applicable to high pressure phase equilibrium and Appendix E contains additional figures and tables not included in the body of the thesis.
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2. BINARY PHASE DIAGRAMS AND THE CRITICAL REGION
The aim of this chapter is to address project objective 1 from Section 1.3 by investigating the unique characteristics of a supercritical fluid (SCF) and to gain an understanding of binary phase diagrams and expected phase behaviour for systems of relevance to SFE applications. The 5 major types of binary phase diagrams as classified by Von Konynburg and Scott [30] are discussed. Particular difficulties in obtaining property values approaching the critical region are then discussed and a computational method is proposed for addressing these challenges.
2.1 The supercritical phase
Some general theory regarding the critical point is firstly presented, followed by a look at the physical fluid properties approaching the critical region.
2.1.1 General critical point theory
Stability and critical point conditions
Criteria for locating a critical point are found by investigating the limit of stability of single homogenous phases [31]. In Sections 6.1 and 8.1 it is shown how the Gibbs energy function (G) is minimized at equilibrium, and how a criterion for equilibrium can be derived from this fact in terms of equality of fugacities, which can be obtained directly from an EOS.
Even though equality of fugacities is a necessary condition for phase equilibrium, it is not sufficient to guarantee a global minimum in the Gibbs energy surface. This requires that the matrix of second derivatives of G with respect to independent composition variables be positive definite, meaning that the thermodynamic surface lies above its tangent plane and has positive curvature [32]. The classic criterion for this limit of stability was given by Gibbs: